Method and apparatus for monitoring a metal layer during chemical mechanical polishing

ABSTRACT

A sensor for monitoring a conductive film in a substrate during chemical mechanical polishing generates an alternating magnetic field that impinges a substrate and induces eddy currents. The sensor can have a core, a first coil wound around a first portion of the core and a second coil wound around a second portion of the core. The sensor can be positioned on a side of the polishing surface opposite the substrate. The sensor can detect a phase difference between a drive signal and a measured signal.

BACKGROUND

The present invention relates generally to chemical mechanical polishingof substrates, and more particularly to methods and apparatus formonitoring a metal layer during chemical mechanical polishing.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. One fabrication step involves depositing a fillerlayer over a non-planar surface, and planarizing the filler layer untilthe non-planar surface is exposed. For example, a conductive fillerlayer can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. The filler layer is thenpolished until the raised pattern of the insulative layer is exposed.After planarization, the portions of the conductive layer remainingbetween the raised pattern of the insulative layer form vias, plugs andlines that provide conductive paths between thin film circuits on thesubstrate. In addition, planarization is needed to planarize thesubstrate surface for photolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing disk pad or beltpad. The polishing pad can be either a “standard” pad or afixed-abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry,including at least one chemically-reactive agent, and abrasive particlesif a standard pad is used, is supplied to the surface of the polishingpad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Overpolishing (removing too much) of a conductive layer orfilm leads to increased circuit resistance. On the other hand,underpolishing (removing too little) of a conductive layer leads toelectrical shorting. Variations in the initial thickness of thesubstrate layer, the slurry composition, the polishing pad condition,the relative speed between the polishing pad and the substrate, and theload on the substrate can cause variations in the material removal rate.These variations cause variations in the time needed to reach thepolishing endpoint. Therefore, the polishing endpoint cannot bedetermined merely as a function of polishing time.

One way to determine the polishing endpoint is to remove the substratefrom the polishing surface and examine it. For example, the substratecan be transferred to a metrology station where the thickness of asubstrate layer is measured, e.g., with a profilometer or a resistivitymeasurement. If the desired specifications are not met, the substrate isreloaded into the CMP apparatus for further processing. This is atime-consuming procedure that reduces the throughput of the CMPapparatus. Alternatively, the examination might reveal that an excessiveamount of material has been removed, rendering the substrate unusable.

More recently, in-situ monitoring of the substrate has been performed,e.g., with optical or capacitance sensors, in order to detect thepolishing endpoint. Other proposed endpoint detection techniques haveinvolved measurements of friction, motor current, slurry chemistry,acoustics and conductivity. One detection technique that has beenconsidered is to induce an eddy current in the metal layer and measurethe change in the eddy current as the metal layer is removed.Unfortunately, the proposed eddy current sensing techniques typicallyrequire complex electronics. In addition, the sensors are positioned onthe backside of the substrate. Since the magnetic field of the sensorextends toward the platen, special shielding is needed to prevent themetal platen from interfering with the eddy current measurements.

SUMMARY

In one aspect, the invention is directed to a sensor for monitoring aconductive film in a substrate. The sensor has a core positionable inproximity to the substrate, a first coil wound around a first portion ofthe core, an oscillator electrically coupled to the first coil to inducean alternating current in the first coil and generate an alternatingmagnetic field in proximity to the substrate, and a second coil woundaround a second portion of the core. A capacitor is electrically coupledto the second coil, and an amplifier is electrically coupled to thesecond coil and the capacitor to generate an output signal.

Implementations of the invention may include one or more of thefollowing features. The oscillator may induce an alternating currentwith a frequency selected to provide a resonant frequency when thesubstrate is not in proximity to the core. The core may consistsessentially of ferrite, and may includes two prongs and a connectingportion between the two prongs. The first coil may be wound around theconnecting portion, and the second coil may be wound around at least oneof the two prongs. The second coil and the capacitor may be connected inparallel. The sensor may be positioned on a side of a polishing padopposite the substrate. The polishing pad may includes an upper layerand a lower layer, and an aperture may be formed in at least a portionof the lower layer adjacent the core. A computer may receive the outputsignal.

In another aspect, the invention is directed to a chemical mechanicalpolishing apparatus. The apparatus has a polishing pad, a carrier tohold a substrate against a first side of the polishing surface, an eddycurrent sensor, and a motor coupled to at least one of the polishing padand carrier head for generating relative motion therebetween. The sensorincludes at least one inductor positioned on a second side of thepolishing pad opposite the substrate, an oscillator electrically coupledto the at least one inductor to induce an alternating current in thecoil and generate an alternating magnetic field, and a capacitorelectrically coupled to the at least one inductor.

Implementations of the invention may include one or more of thefollowing features. A platen may support the polishing pad, and the atleast one inductor may be positioned in a recess in a top surface of theplaten. The platen may rotates, and a position sensor may determine anangular position of the platen and a controller to sample data from theeddy current sensor when the at least one inductor is positionedadjacent the substrate. A recess may be formed in the second side of thepolishing pad. The polishing pad may include a cover layer on the firstside of the polishing pad and a backing layer on the second side of thepolishing pad, and the recess may be formed by removing a portion of thebacking layer. The eddy current sensor may include a core having twopoles positioned adjacent the recess in the polishing pad, and the atleast one inductor is wound around a first portion of the core. The eddycurrent sensor may include a core, and the at least one inductor mayinclude a first inductor wound around a first portion of the core and asecond inductor wound around a second portion of the core. Theoscillator may be electrically coupled to the first coil to induce analternating current in the first coil. The capacitor may be electricallycoupled to the second coil. The oscillator may induce an alternatingcurrent with a frequency selected to provide a resonant frequency whenthe substrate is not in proximity to the core. An endpoint detectionsystem may receive an output signal from the eddy current sensor. Theendpoint detection system may be configured to signal a polishingendpoint if the output signal exceeds a predetermined threshold.

In another aspect, the invention may be directed to a method ofmonitoring a thickness of a conductive layer in a substrate during apolishing operation. In the method, a substrate is positioned on a firstside of a polishing surface, and an alternating magnetic field isgenerated from an inductor positioned on a second side of the polishingsurface opposite the substrate. The magnetic field extends through thepolishing surface to induce eddy currents in the conductive layer. Achange in the alternating magnetic field caused by a change in thethickness of the conductive layer is detected.

Implementations of the invention may include one or more of thefollowing features. A first coil may be driven with an oscillator at afirst frequency. The first frequency may be a resonant frequency whenthe substrate is not in proximity to the magnetic field. The alternatingmagnetic field may be sensed with a second coil. The second coil may beconnected in parallel with a capacitor. The first coil may be woundaround a first portion of a core, and the second coil may be woundaround a second portion of the core. When the inductor is adjacent thesubstrate may be determined. The inductor may be driven with a firstsignal, and a second signal may be generated from the alternatingmagnetic field. A change in amplitude in the second signal may bedetermined. A change in a phase difference between the first signal andthe second signal may be determined.

In another aspect, the invention is directed to a method of chemicalmechanical polishing. In the method, a substrate having a conductivelayer is positioned on a first side of the polishing surface. Analternating magnetic field is generated from an inductor positioned on asecond side of the polishing surface opposite the substrate. Themagnetic field extends through the polishing surface to induce eddycurrents in the conductive layer. Relative motion is created between thesurface and the polishing surface to polish the conductive layer. Theeddy currents in the substrate are sensed, and polishing is halted whenthe sensed eddy currents exhibit an endpoint criteria.

Implementations of the invention may include one or more of thefollowing features. The endpoint criteria may be the eddy currentspassing a threshold strength or leveling off.

In another aspect, the invention is directed to a chemical mechanicalpolishing apparatus. The apparatus has a polishing pad with a polishingsurface, a carrier to hold a substrate against the polishing surface, amotor coupled to at least one of the polishing pad and carrier head forgenerating relative motion therebetween, and a conductive layerthickness monitoring system. The conductive layer thickness monitoringsystem including at least one inductor, a current source that generatesa drive signal, the current source electrically coupled to the at leastone inductor to induce an alternating current in the at least oneinductor and generate an alternating magnetic field, sense circuitryincluding a capacitor electrically coupled to the at least one inductorto sense the alternating magnetic field and generate a sense signal, andphase comparison circuitry coupled to the current source and the sensecircuitry to measure a phase difference between the sense signal and thedrive signal.

Implementations of the invention may include one or more of thefollowing features. At least one first gate, e.g., an XOR gate, mayconvert sinusoidal signals from the inductor and the oscillator intofirst and second square-wave signals. A comparator, e.g., an XOR gate,may compare the first square-wave signal to the second square-wavesignal to generate a third square-wave signal. A filter may convert thethird square-wave signal into differential signal having an amplitudeproportional to the phase difference between the first and second squarewave signals. The phase comparison circuitry may generate a signal witha duty cycle proportional to the phase difference.

In another aspect, the invention may be directed to a method ofmonitoring a thickness of a conductive layer on a substrate during achemical mechanical polishing operation. In the method, a coil isenergized with a first signal to generate an alternating magnetic field.The alternating magnetic field induces eddy currents in a conductivelayer of the substrate. The alternating magnetic field is measured and asecond signal is generated indicative of the magnetic field. Te firstand second signals are compared to determine a phase differencetherebetween.

Implementations of the invention can include zero or more of thefollowing possible advantages. The endpoint detector can sense thepolishing endpoint of a metal layer in-situ. The magnetic fieldapparatus for the endpoint detector can be embedded in the platen belowa polishing pad. The magnetic field apparatus can be protected frompolishing environment, e.g., corrosive slurry. The endpoint detectorneed not use complex electronics. Polishing can be stopped withreasonable accuracy. Overpolishing and underpolishing substrate can bereduced, thereby improving yield and throughput.

Other features and advantages of the invention will become apparent fromthe following description, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic exploded perspective view of a chemical mechanicalpolishing apparatus.

FIG. 2 is a schematic side view, partially cross-sectional, of achemical mechanical polishing apparatus including an eddy currentmonitoring system.

FIG. 3 is a schematic circuit diagram of the eddy current monitoringsystem.

FIGS. 4A-4C are schematic cross-sectional views of a polishing pad.

FIG. 5 is a schematic cross-sectional view illustrating a magnetic fieldgenerated by the monitoring system.

FIG. 6 is a schematic perspective view of a core from an eddy currentsensor.

FIGS. 7A-7D schematically illustrating a method of detecting a polishingendpoint using an eddy current sensor.

FIG. 8 is a graph illustrating a trace from the eddy current monitoringsystem.

FIG. 9 is a schematic diagrams an eddy current monitoring system thatsenses a phase shift.

FIGS. 10A and 10B are schematic circuit diagrams of two implementationsof an eddy current monitoring system of FIG. 9.

FIG. 11 is a graph illustrating a trace from the eddy current monitoringsystem that measures phase shift.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2A, one or more substrates 10 can be polishedby a CMP apparatus 20. A description of a similar polishing apparatus 20can be found in U.S. Pat. No. 5,738,574, the entire disclosure of whichis incorporated herein by reference. Polishing apparatus 20 includes aseries of polishing stations 22 and a transfer station 23. Transferstation 23 transfers the substrates between the carrier heads and aloading apparatus.

Each polishing station includes a rotatable platen 24 on which is placeda polishing pad 30. The first and second stations can include atwo-layer polishing pad with a hard durable outer surface or afixed-abrasive pad with embedded abrasive particles. The final polishingstation can include a relatively soft pad. Each polishing station canalso include a pad conditioner apparatus 28 to maintain the condition ofthe polishing pad so that it will effectively polish substrates.

A two-layer polishing pad 30 typically has a backing layer 32 whichabuts the surface of platen 24 and a covering layer 34 which is used topolish substrate 10. Covering layer 34 is typically harder than backinglayer 32. However, some pads have only a covering layer and no backinglayer. Covering layer 34 can be composed of foamed or cast polyurethane,possibly with fillers, e.g., hollow microspheres, and/or a groovedsurface. Backing layer 32 can be composed of compressed felt fibersleached with urethane. A two-layer polishing pad, with the coveringlayer composed of IC-1000 and the backing layer composed of SUBA-4, isavailable from Rodel, Inc., of Newark, Del. (IC-1000 and SUBA-4 areproduct names of Rodel, Inc.).

A rotatable multi-head carousel 60 supports four carrier heads 70. Thecarousel is rotated by a central post 62 about a carousel axis 64 by acarousel motor assembly (not shown) to orbit the carrier head systemsand the substrates attached thereto between polishing stations 22 andtransfer station 23. Three of the carrier head systems receive and holdsubstrates, and polish them by pressing them against the polishing pads.Meanwhile, one of the carrier head systems receives a substrate from anddelivers a substrate to transfer station 23.

Each carrier head 70 is connected by a carrier drive shaft 74 to acarrier head rotation motor 76 (shown by the removal of one quarter ofcover 68) so that each carrier head can independently rotate about itown axis. In addition, each carrier head 70 independently laterallyoscillates in a radial slot 72 formed in carousel support plate 66. Adescription of a suitable carrier head 70 can be found in U.S. patentapplication Ser. No. 09/470,820, filed Dec. 23, 1999, the entiredisclosure of which is incorporated by reference. In operation, theplaten is rotated about its central axis 25, and the carrier head isrotated about its central axis 71 and translated laterally across thesurface of the polishing pad.

A slurry 38 containing a reactive agent (e.g., deionized water for oxidepolishing) and a chemically-reactive catalyzer (e.g., potassiumhydroxide for oxide polishing) can be supplied to the surface ofpolishing pad 30 by a slurry supply port or combined slurry/rinse arm39. If polishing pad 30 is a standard pad, slurry 38 can also includeabrasive particles (e.g., silicon dioxide for oxide polishing).

Referring to FIGS. 2A and 3, a recess 26 is formed in platen 24, and athin section 36 can be formed in polishing pad 30 overlying recess 26.Aperture 26 and thin pad section 36, if needed, are positioned such thatthey pass beneath substrate 10 during a portion of the platen'srotation, regardless of the translational position of the carrier head.Assuming that polishing pad 32 is a two-layer pad, thin pad section 36can be constructed as shown in FIG. 4A by removing a portion 33 ofbacking layer 32. Alternatively, as shown in FIG. 4B, thin pad section36′ can be formed by removing a portion 33′ of both backing layer 32′and a portion of cover layer 34′. Thus, this implementation has a recessin the bottom surface of cover layer 34 in the thin pad section 36. Ifthe polishing pad is a single-layer pad, thin pad section 36 can beformed by removing a portion of the pad material to create a recess inthe bottom surface of the pad. Alternatively, as shown in FIG. 4C, thinpad section 36″ can be formed by inserting a plug 37 of a differentmaterial into polishing pad 30. For example, the plug can be arelatively pure polymer or polyurethane, e.g., formed without fillers.In general, the material of pad section 36 should be non-magnetic ornon-conductive. If the polishing pad is itself sufficiently thin or hasa magnet permeability (and conductivity) that does not interfere withthe eddy current measurements, then the pad does not need anymodifications or recesses.

Returning to FIGS. 2A and 3, an in-situ eddy current monitoring system40, which can function as an endpoint detector, includes a drive system48 to induce eddy currents in a metal layer on the substrate and asensing system 58 to detect eddy currents induced in the metal layer bythe drive system. The monitoring system 40 includes a core 42 positionedin recess 26 to rotate with the platen, a drive coil 44 wound around onepart of core 42, and a sense coil 46 wound around second part of core42. For drive system 48, monitoring system 40 includes an oscillator 50connected to drive coil 44. For sense system 58, monitoring system 40includes a capacitor 52 connected in parallel with sense coil 46, an RFamplifier 54 connected to sense coil 46, and a diode 56. The oscillator50, capacitor 52, RF amplifier 54, and diode 56 can be located apartfrom platen 24, and can be coupled to the components in the platenthrough a rotary electrical union 29.

Referring to FIG. 5, in operation the oscillator 50 drives drive coil 44to generate an oscillating magnetic field 48 that extends through thebody of core 42 and into the gap 46 between the two poles 42 a and 42 bof the core. At least a portion of magnetic field 48 extends throughthin portion 36 of polishing pad 30 and into substrate 10. If a metallayer 12 is present on substrate 10, oscillating magnetic field 48generates eddy currents in the metal layer 12. The eddy currents causethe metal layer 12 to act as an impedance source in parallel with sensecoil 46 and capacitor 52. As the thickness of the metal layer changes,the impedance changes, resulting in a change in the Q-factor of sensingmechanism. By detecting the change in the Q-factor of the sensingmechanism, the eddy current sensor can sense the change in the strengthof the eddy currents, and thus the change in thickness of metal layer12.

Referring to FIG. 6, core 42 can be a U-shaped body formed of anon-conductive material with a relatively high magnetic permeability(e.g., of about 2500). Specifically, core 42 can be ferrite. In oneimplementation, the two poles 42 a and 42 b are about 0.6 inches apart,the core is about 0.6 inches deep, and the cross-section of the core isa square about 0.2 inches on a side.

In general, the in-situ eddy current monitoring system 40 is constructedwith a resonant frequency of about 50 kHz to 10 MHz, e.g., 2 MHz. Forexample, the sense coil 46 can have an inductance of about 0.3 to 30 Hand the capacitor 52 can have a capacitance of about 0.2 to 20 nF. Thedriving coil can be designed to match the driving signal fromoscillator. For example, if the oscillator has a low voltage and a lowimpedance, the drive coil can include fewer turns to provide a smallinductance. On the other hand, if the oscillator has a high voltage anda high impedance, the drive coil can include more turns to provide alarge inductance.

In one implementation, the sense coil 46 includes nine turns around eachprong of the core, and the drive coil 44 includes two turns around thebase of the core, and the oscillator drives the drive coil 44 with anamplitude of about 0.1 V to 5.0 V. Also, in one implementation, thesense coil 46 has an inductance of about 2.8 H, the capacitor 52 has acapacitance of about 2.2 nF, and the resonant frequency is about 2 MHz.In another implementation, the sense coil has an inductance of about 3 Hand the capacitor 52 has a capacitance of about 400 pF. Of course, thesevalues are merely exemplary, as they are highly sensitive to the exactwinding configuration, core composition and shape, and capacitor size.

In general, the greater the expected initial thickness of the conductivefilm, the lower the desired resonant frequency. For example, for arelatively thin film, e.g., 2000 Angstroms, the capacitance andinductance can be selected to provide a relatively high resonantfrequency, e.g., about 2 MHz. On the other hand, for a relativelythicker film, e.g., 2000 Angstroms, the capacitance and inductance canbe selected to provide a relatively lower resonant frequency, e.g.,about 50 kHz. However, high resonant frequencies may still work wellwith thick copper layers. In addition, very high frequencies (above 2MHz) can be used to reduce background noise from metal parts in thecarrier head.

Returning to FIGS. 2A, 2B and 3, the CMP apparatus 20 can also include aposition sensor 80, such as an optical interrupter, to sense when core42 is beneath substrate 10. For example, the optical interrupter couldbe mounted at a fixed point opposite carrier head 70. A flag 82 isattached to the periphery of the platen. The point of attachment andlength of flag 82 is selected so that it interrupts the optical signalof sensor 80 while core 42 sweeps beneath substrate 10. Alternately, theCMP apparatus can include an encoder to determine the angular positionof platen.

In operation, CMP apparatus 20 uses monitoring system 40 to determinewhen the bulk of the filler layer has been removed and the underlyingstop layer has been exposed. Monitoring system 40 can as be used todetermine the amount of material removed from the surface of thesubstrate. A general purpose programmable digital computer 90 can beconnected to amplifier 56 to receive the intensity signal from the eddycurrent sensing system. Computer 90 can be programmed to sampleamplitude measurements from the monitoring system when the substrategenerally overlies the core, to store the amplitude measurements, and toapply the endpoint detection logic to the measured signals to detect thepolishing endpoint. Possible endpoint criteria for the detector logicinclude local minima or maxima, changes in slope, threshold values inamplitude or slope, or combinations thereof.

Referring to FIG. 2B, the core 42, drive coil 44 and sense coil 46 ofthe eddy current sensor located below thin section 36 of polishing pad32 sweep beneath the substrate with each rotation of the platen.Therefore, the computer 90 can also be programmed to divide theamplitude measurements from each sweep of the core beneath the substrateinto a plurality of sampling zones 96, to calculate the radial positionof each sampling zone, to sort the amplitude measurements into radialranges, to determine minimum, maximum and average amplitude measurementsfor each sampling zone, and to use multiple radial ranges to determinethe polishing endpoint, as discussed in U.S. patent application Ser. No.09/460,529, filed Dec. 13, 1999, the entirety of which is incorporatedherein by reference.

Since the eddy current sensor sweeps beneath the substrate with eachrotation of the platen, information on the metal layer thickness isbeing accumulated in-situ and on a continuous real-time basis. In fact,the amplitude measurements from the eddy current sensor can be displayedon an output device 92 during polishing to permit the operator of thedevice to visually monitor the progress of the polishing operation.

Moreover, after sorting the amplitude measurements into radial ranges,information on the metal film thickness can be fed in real-time into aclosed-loop controller to periodically or continuously modify thepolishing pressure profile applied by a carrier head, as discussed inU.S. Patent application Ser. No. 60/143,219, filed Jul. 7, 1999, theentirety of which is incorporated herein by reference. For example, thecomputer could determine that the endpoint criteria have been satisfiedfor the outer radial ranges but not for the inner radial ranges. Thiswould indicate that the underlying layer has been exposed in an annularouter area but not in an inner area of the substrate. In this case, thecomputer could reduce the diameter of the area in which pressure isapplied so that pressure is applied only to the inner area of thesubstrate, thereby reducing dishing and erosion on the outer area of thesubstrate. Alternatively, the computer can halt polishing of thesubstrate on the first indication that the underlying layer has beenexposed anywhere on the substrate, i.e., at first clearing of the metallayer.

Initially, referring to FIGS. 2A, 3 and 7A, oscillator 50 is tuned tothe resonant frequency of the LC circuit, without any substrate present.This resonant frequency results in the maximum amplitude of the outputsignal from RF amplifier 54.

As shown in FIGS. 7B and 8, for a polishing operation, a substrate 10 isplaced in contact with polishing pad 30. Substrate 10 can include asilicon wafer 12 and a conductive layer 16, e.g., a metal such ascopper, disposed over one or more patterned underlying layers 14, whichcan be semiconductor, conductor or insulator layers. The patternedunderlying layers can include metal features, e.g., vias, pads andinterconnects. Since, prior to polishing, the bulk of conductive layer16 is initially relatively thick and continuous, it has a lowresistivity, and relatively strong eddy currents can be generated in theconductive layer. As previously mentioned, the eddy currents cause themetal layer to function as an impedance source in parallel with sensecoil 46 and capacitor 52. Consequently, the presence of conductive film16 reduces the Q-factor of the sensor circuit, thereby significantlyreducing the amplitude of the signal from RF amplifier 56.

Referring to FIGS. 7C and 8, as substrate 10 is polished, the bulkportion of conductive layer 16 is thinned. As the conductive layer 16thins, its sheet resistivity increases, and the eddy currents in themetal layer become dampened. Consequently, the coupling between metallayer 16 and sensor circuitry 58 is reduced (i.e., increasing theresistivity of the virtual impedance source). As the coupling declines,the Q-factor of the sensor circuit 58 increases toward its originalvalue.

Referring to FIGS. 7D and 8, eventually the bulk portion of conductivelayer 16 is removed, leaving conductive interconnects 16′ in thetrenches between the patterned insulative layer 14. At this points, thecoupling between the conductive portions in the substrate, which aregenerally small and generally non-continuous, and sensor circuit 58reaches a minimum. Consequently, the Q-factor of the sensor circuitreaches a maximum value (although not as large as the Q-factor when thesubstrate is entirely absent). This causes the amplitude of the outputsignal from the sensor circuit to plateau. Thus, by sensing when theamplitude of the output signal is no longer increasing and has leveledoff (e.g., reached a local plateau), computer 90 can sense a polishingendpoint. Alternatively, by polishing one or more test substrates, theoperator of the polishing machine can determine the amplitude of theoutput signal as a function of the thickness of the metal layer. Thus,the endpoint detector can halt polishing when a particular thickness ofthe metal layer remains on the substrate. Specifically, computer 90 cantrigger the endpoint when the output signal from the amplifier exceeds avoltage threshold corresponding to the desired thickness.

The eddy current monitoring system can also be used to trigger a changein polishing parameters. For example, when the monitoring system detectsa polishing criterion, the CMP apparatus can change the slurrycomposition (e.g., from a high-selectivity slurry to a low selectivityslurry). As another example, as discussed above, the CMP apparatus canchange the pressure profile applied by the carrier head.

In addition to sensing changes in amplitude, the eddy current monitoringsystem can calculate a phase shift in the sensed signal. As the metallayer is polished, the phase of the sensed signal changes relative tothe drive signal from the oscillator 50. This phase difference can becorrelated to the thickness of the polished layer. One implementation ofa phase measuring device, shown in FIG. 10A, combines the drive andsense signals to generate a phase shift signal with a pulse width orduty cycle which is proportional to the phase difference. In thisimplementation, two XOR gates 100 and 102 are used to convert sinusoidalsignals from the sense coil 46 and oscillator 50, respectively, intosquare-wave signals. The two square-wave signals are fed into the inputsof a third XOR gate 104. The output of the third XOR gate 104 is a phaseshift signal with a pulse width or duty cycle proportional to the phasedifference between the two square wave signals. The phase shift signalis filtered by an RC filter 106 to generate a DC-like signal with avoltage proportional to the phase difference. Alternatively, the signalscan be fed into a programmable digital logic, e.g., a ComplexProgrammable Logic Device (CPLD) or Field Programmable Gate Array (FGPA)that performs the phase shift measurements.

The phase shift measurement can be used to detect the polishing endpointin the same fashion as the amplitude measurements discussed above.Alternatively, both amplitude and phase shift measurements could be usedin the endpoint detection algorithm. An implementation for both theamplitude and phase shift portions of the eddy current monitoring systemis shown in FIG. 10A. An implementation of the amplitude sensing portionof the eddy current monitoring system is shown in FIG. 10B. An exampleof a trace generated by an eddy current monitoring system that measuresthe phase difference between the drive and sense signals is shown inFIG. 11. Since the phase measurements are highly sensitive to thestability of the driving frequency, phase locked loop electronics may beadded.

A possible advantage of the phase difference measurement is that thedependence of the phase difference on the metal layer thickness may bemore linear than that of the amplitude. In addition, the absolutethickness of the metal layer may be determined over a wide range ofpossible thicknesses.

The eddy current monitoring system can be used in a variety of polishingsystems. Either the polishing pad, or the carrier head, or both can moveto provide relative motion between the polishing surface and thesubstrate. The polishing pad can be a circular (or some other shape) padsecured to the platen, a tape extending between supply and take-uprollers, or a continuous belt. The polishing pad can be affixed on aplaten, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there could be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather thantuning when the substrate is absent, the drive frequency of theoscillator can be tuned to a resonant frequency with a polished orunpolished substrate present (with or without the carrier head), or tosome other reference.

Various aspects of the invention, such as placement of the coil on aside of the polishing surface opposite the substrate or the measurementof a phase difference, still apply if the eddy current sensor uses asingle coil. In a single coil system, both the oscillator and the sensecapacitor (and other sensor circuitry) are connected to the same coil.

The present invention has been described in terms of a preferredembodiment. The invention, however, is not limited to the embodimentdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

1. A chemical mechanical polishing apparatus, comprising: a polishingpad; a carrier to hold a substrate against a polishing surface of thepolishing pad; a sensor for monitoring a conductive film in a substrate,the sensor including a core positionable in proximity to the substrate,a first coil wound around a first portion of the core, an oscillatorelectrically coupled to the first coil to induce an alternating currentin the first coil and generate an alternating magnetic field inproximity to the substrate, a second coil wound around a second portionof the core, a capacitor electrically coupled to the second coil, and anamplifier electrically coupled to the second coil and the capacitor togenerate an output signal, wherein the oscillator induces an alternatingcurrent with a substantially constant frequency selected to provide aresonant frequency when the substrate is not in proximity to the core; amotor coupled to at least one of the polishing pad and carrier head forgenerating relative motion therebetween; and a computer to receive andmonitor the output signal from the sensor while the substrate contactsthe polishing pad and the oscillator induces the alternating currentwith the substantially constant frequency.
 2. The apparatus of claim 1,wherein the core consists essentially of ferrite.
 3. The apparatus ofclaim 1, wherein the core includes two prongs and a connecting portionbetween the two prongs.
 4. The apparatus of claim 3, wherein the firstcoil is wound around the connecting portion and the second coil is woundaround at least one of the two prongs.
 5. The apparatus of claim 1,wherein the second coil and the capacitor are connected in parallel. 6.The apparatus of claim 1, wherein the sensor is positioned on a side ofa polishing pad opposite the substrate.
 7. The apparatus of claim 1,wherein the eddy current sensor includes a core having two polespositioned adjacent the recess in the polishing pad, and the at leastone inductor is wound around a first portion of the core.
 8. A chemicalmechanical polishing apparatus, comprising: a polishing pad having apolishing surface; a carrier to hold a substrate against the polishingsurface; an eddy current sensor including at least one inductor, anoscillator electrically coupled to the at least one inductor to inducean alternating current in the at least one inductor and generate analternating magnetic field, and a capacitor electrically coupled to theat least one inductor; a motor coupled to at least one of the polishingpad and carrier head for generating relative motion therebetween; and aplaten to support the polishing pad, and wherein the platen rotates, andfurther comprising a position sensor to determine an angular position ofthe platen and a controller to sample data from the eddy current sensorwhen the at least one inductor is positioned adjacent the substrate. 9.The apparatus of claim 8, wherein the inductor comprises at least onecoil wound around a core, and wherein when the substrate is in proximityto the core, the oscillator induces the alternating current with afrequency selected to provide a resonant frequency when the substrate isnot in proximity to the core.
 10. The apparatus of claim 8, furthercomprising an endpoint detection system to receive an output signal fromthe eddy current sensor, the endpoint detection system configured tosignal a polishing endpoint if the output signal exceeds a predeterminedthreshold.
 11. The apparatus of claim 8, wherein the eddy current sensorincludes a core and the at least one inductor is positioned to generatethe magnetic field in the core.
 12. The apparatus of claim 11, whereinthe core consists essentially of ferrite.
 13. The apparatus of claim 11,wherein the core includes two prongs and a connecting portion betweenthe two prongs.
 14. The apparatus of claim 11, wherein the at least oneinductor includes a first coil wound around a first portion of the coreand a second coil wound around a second portion of the core.
 15. Theapparatus of claim 14, wherein the oscillator is electrically coupled tothe first coil to induce an alternating current in the first coil. 16.The apparatus of claim 14, wherein the capacitor is electrically coupledto the second coil.
 17. A chemical mechanical polishing apparatus,comprising: a polishing pad having a polishing surface; a carrier tohold a substrate against the polishing surface; an eddy current sensorincluding a core, at least one inductor positioned on a side of thepolishing surface opposite the substrate, the at least one inductorincluding a first inductor wound around a first portion of the core anda second inductor wound around a second portion of the core, anoscillator electrically coupled to the at least one inductor to inducean alternating current in the at least one inductor and generate analternating magnetic field, and a capacitor electrically coupled to theat least one inductor; and a motor coupled to at least one of thepolishing pad and carrier head for generating relative motiontherebetween, wherein the oscillator is electrically coupled to thefirst inductor to induce an alternating current in the first inductor,and wherein the capacitor is electrically coupled to the second coil.18. A chemical mechanical polishing apparatus, comprising: a polishingpad; a carrier to hold a substrate against a polishing surface of thepolishing pad on the support; an eddy current sensor including at leastone inductor positioned to be on a side of the polishing surfaceopposite the substrate when the polishing pad is on the support, anoscillator electrically coupled to the at least one inductor to inducean alternating current in the coil at least one inductor and generate analternating magnetic field; a motor coupled to at least one of thesupport and the carrier head for generating relative motiontherebetween; and further comprising a position sensor to determine anangular position of the platen and a controller to sample data from theeddy current sensor when the at least one inductor is positionedadjacent the substrate.
 19. The apparatus of claim 18, wherein thesensor includes a core and the at least one inductor is positioned togenerate the magnetic field in the core.
 20. The apparatus of claim 18,wherein the polishing pad support comprises a rotatable platen.
 21. Theapparatus of claim 18, further comprising an endpoint detection systemto receive an output signal from the eddy current sensor and determine apolishing endpoint.
 22. The apparatus of claim 18, wherein the eddycurrent sensor includes a core having two poles, and the at least oneinductor is wound around a first portion of the core.
 23. The apparatusof claim 18, wherein the eddy current sensor includes a core and the atleast one inductor is positioned to generate the magnetic field in thecore.
 24. The apparatus of claim 23, wherein the core consistsessentially of ferrite.
 25. The apparatus of claim 23, wherein the coreincludes two prongs and a connecting portion between the two prongs. 26.The apparatus of claim 23, wherein the at least one inductor includes afirst coil wound around a first portion of the core and a second coilwound around a second portion of the core.
 27. The apparatus of claim26, wherein the oscillator is electrically coupled to the first coil toinduce an alternating current in the first coil.
 28. The apparatus ofclaim 26, further comprising a capacitor electrically coupled to thesecond coil.